Magnetic resonance (MR) imaging is a known technology that can produce images of the inside of an examination subject without radiation exposure. In a typical MR imaging procedure, the subject is positioned in a strong, static, homogeneous base magnetic field B0 (having a field strength that is typically between about 0.5 Tesla and 3 Tesla) in an MR apparatus, so that the subject's nuclear spins become oriented along the base magnetic field.
Radio-frequency (RF) excitation pulses are directed into the examination subject to excite nuclear magnetic resonances, and subsequent relaxation of the excited nuclear magnetic resonances can generate RF signals. Rapidly switched magnetic gradient fields can be superimposed on the base magnetic field, in various orientations, to provide spatial coding of the RF signal data (also referred to as image data). The RF signal data can be detected during a ‘readout’ phase, and mathematically processed to reconstruct images of the examination subject. For example, the acquired RF signal data are typically digitized and stored as complex numerical values in a k-space matrix. An associated MR image can be reconstructed from the k-space matrix populated with such values using a multi-dimensional Fourier transformation.
A magnetic resonance imaging technique known as echo planar imaging (EPI) is based on generation of RF signal data (or MR image data) in rapid sequence as a plurality of gradient echoes in the readout phase, with a rapid changing of the polarity of a readout gradient for successive echo signals. The alternating polarity of the readout gradients result in raw image data that fills lines in k-space in alternate directions (e.g., right-to-left, then left-to-right, then right-to-left, etc.). For example, an EPI sequence can include an RF excitation pulse and an image readout sequence that includes the gradient echoes accompanied by appropriate phase encoding and readout gradients.
EPI sequences used to image a plurality of slices can be prone to certain effects that can affect the resulting image data and lead to artifacts and/or degradation in the resulting image. For example, even slight deviations in phase that may occur when obtaining successive rows of data in k-space can lead to an artifact known as N/2 ghosting. An N/2 ghost can appear in the final image as “ghost” images, having a different intensity than the “true” image and shifted by N/2 in the positive and negative directions with reference to the middle of the N×N image matrix.
Another source of image degradation that can occur during EPI imaging sequences arises from slight changes in the local primary magnetic field B0 during imaging (also known as B0 drift). Such changes can result, for example, from heating or vibration in the imaging hardware during an imaging sequence. B0 drift can lead to such artifacts as loss of contrast, banding, or the like in different imaging sequences.
To correct for these undesirable effects, conventional non-simultaneous multislice EPI sequences often include acquisition of a set of navigator echoes prior to acquiring the imaging data for each slice. Such navigator echoes provide one-dimensional image information that can be used to identify and correct for, e.g., the aforementioned effects.
Navigator image data that can be used to improve image data quality for EPI imaging may include three non-phase-encoded echoes acquired with positive and negative readout gradient polarities. These navigator echoes with opposite readout gradient polarities can be referred to as odd and even echoes. An exemplary navigator sequence that includes a series of three echoes with alternating negative-positive-negative (even-odd-even) readout gradients is shown in the left half of FIG. 2A, preceding the conventional EPI readout sequence shown on the right half of FIG. 2A. This exemplary navigator sequence can be used to correct both N/2 ghosting and B0 drift effects during image reconstruction as follows.
For slice-specific N/2 ghost correction, the 0th and 1st order phase shifts between odd and even echoes can be determined in the readout direction by correlating their image space data. During image reconstruction, these factors are then used to retrospectively realign the odd and even k-space lines in order to compensate for shifts caused by factors such as gradient delays and eddy currents. This correction procedure is described, e.g., in U.S. Pat. No. 6,043,651 to Heid, which is incorporated herein by reference in its entirety.
B0 field drift effects can be corrected using a technique based on dynamic off-resonance in k-space (DORK) as described, e.g., in US Patent Publication No. US 2012/0249138A1 by Pfeuffer, which is incorporated herein by reference in its entirety. In the DORK method, the temporal B0 field drift is determined by comparing the phase evolution of navigator echoes having identical readout gradient polarities between consecutively acquired image volumes. Typically, this calculation is performed as an average over a complete image volume to improve accuracy.
In simultaneous multislice (SMS) EPI, several slices are excited and acquired simultaneously, leading to a k-space dataset which is based on signals originating from several slices collapsed on top of each other. Separating (or uncollapsing) of the slice data is performed during image reconstruction with the slice GRAPPA method that is described, e.g., in Setsompop et al., Blipped-Controlled Aliasing in Parallel Imaging for Simultaneous Multislice Echo Planar Imaging with Reduced g-Factor Penalty, Magnetic Resonance in Medicine 67:1210-1224 (2012), and in Setsompop et al., Improving diffusion MRI using simultaneous multi-slice echo planar imaging, NeuroImage 63 (2012), 569-580, both of which are incorporated herein by reference in their entireties.
The slice GRAPPA method is based on uncollapsing the data for each slice using a slice-specific GRAPPA kernel. These kernels are obtained from a separate non-collapsed reference scan that is obtained prior to the series of sequences used to obtain the collapsed image data. To reduce artifacts that may arise from misalignment of even and odd echoes in k-space, two kernels (odd and even) are calculated per slice.
For SMS acquisitions, the aforementioned navigator data obtained simultaneously from multiple slices is also collapsed in the same way as the image data. Because the DORK method performs an average calculation over the complete image volume, the collapsed navigator datasets can be used directly in that technique to correct for B0 field drift effects. However, such collapsed multislice navigator cannot be utilized for slice-specific N/2 ghost correction, because there is no known way to uncollapse the non-phase-encoded navigator data, e.g., to generate slice-specific navigator data.
In certain SMS EPI imaging procedures, the non-collapsed navigator data (navigator lines) from the reference scan can be utilized for all imaging volumes in a subsequent series of data acquisitions. However, this approach can lead to increased ghosting artifacts if the conditions under which the reference navigator data were acquired are changed, e.g., by subject motion or eddy current variations during the imaging procedure.
Accordingly, it would be desirable to have a system and method for magnetic resonance imaging that addresses some of the shortcomings described above, and that may further provide navigator data that can be used to correct for both B0 drift and N/2 ghosting during simultaneous multislice EPI imaging procedures.